Fluoropolymers, i.e. polymers having a fluorinated backbone, have been long known and used in a various applications because of their desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. Various fluoropolymers are for example described in “Modern Fluoropolymers”, edited by John Scheirs (ed), Wiley Science 1997. The fluoropolymers may have a partially fluorinated backbone, generally at least 40% by weight fluorinated, or a fully fluorinated backbone. Particular examples of fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP), typically referred to as FEP, perfluoroalkoxy copolymers (PFA), ethylene-tetrafluoroethylene (ETFE) copolymers, terpolymers of tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) and polyvinylidene fluoride polymers (PVDF).
The fluoropolymers may be used to coat or impregnate substrates to provide desirable properties thereto such as for instance chemical resistance, weatherability, water- and oil repellence, lubricity etc. For example, aqueous dispersions of fluoropolymers may be used to coat or impregnate substrates such as metals, fabrics, textiles, glass fibers or paper.
A frequently used method for producing aqueous dispersions of fluoropolymers involves aqueous emulsion polymerization of one or more fluorinated monomers. Usually, one or more concentration steps follow the polymerization reaction to increase the content of solids in the raw dispersion. The aqueous emulsion polymerization of fluorinated monomers generally involves the use of an emulsifier. Typically, the emulsifiers are perfluorinated anionic surfactants. The fluorinated emulsifier stabilises the fluoropolymer in the aqueous medium and prevents the fluoropolymer from coagulating in the dispersion. Typical examples of fluorinated emulsifiers are perfluorinated carboxylic acids, such as, for example, perfluorooctanoic acids and salts thereof, in particular ammonium perfluorooctanoic acid (APFO). Fluorinated emulsifiers are generally expensive compounds and in several cases fluorinated emulsifiers have been found to be poorly biodegradable. Accordingly, measures have been taken to remove fluorinated emulsifiers from the aqueous fluoropolymer dispersions.
WO 00/35971 describes a method in which the amount of fluorinated emulsifier in aqueous dispersions is reduced by contacting the dispersion with an anion exchange resin to which the (anionic) fluorinated emulsifier binds. Non-ionic surfactants are added to the dispersion prior to the ion-exchange to stabilise the fluoropolymer in the dispersion in the absence of the fluorinated emulsifier. The resulting emulsifier-free or emulsifier-reduced fluoropolymer dispersions can be conveniently used in applications where the fluoropolymers are applied to a substrate directly from the dispersion.
However, in certain applications where the fluoropolymer is not applied to a substrate directly from the dispersion, for example, where the fluoropolymer is applied as a coagulum, e.g. as a paste or solid, it may be desirable to avoid or at least to reduce the presence of non-ionic surfactants. In these applications the fluoropolymer is separated from the dispersion prior to application to the substrate which is typically done by destabilising the dispersion and separating the fluoropolymer from the aqueous medium (also referred to as phase-separation or coagulation). Non-ionic surfactants, however, have been observed to prevent or inhibit the phase separation when using common phase separation techniques such as, for example, salting out (i.e. increasing the ionic strength of the dispersion by adding salts or acids), shear force-induced coagulation, or solvent-induced coagulation (e.g. adding organic solvents). Instead of the formation of distinct phases, a fluoropolymer phase and a water phase, slurries are often contained. If the fluoropolymers can be collected from those poorly phase-separated mixtures at all, they typically contain rather large amounts of residual non-ionic surfactants and water, the presence of which impacts on the physical properties of coatings prepared from these poorly phase-separated fluoropolymers. For example, due to the presence of the surfactants the fluoropolymer coating may adsorb water, for instance from ambient humidity, leading to poor properties as regards, for example, surface hardness, water resistance, self-lubrication or friction-resistance etc.